Electromagnetic radiation detectors that are responsive to light in more than one wavelength band, also referred to as multi-spectral or multi-color detectors, provide a number of advantages in modern imaging systems. In general, the light that is detected may be visible light or light that is not visible to the human eye (e.g., infrared (IR) radiation).
Early efforts to detect IR radiation within more than one spectral band have relied on the use of multiple detector arrays, each having a different spectral filter. Multiple detector arrays with different spectral responses have also been used. The use of a continuously variable wedge filter in conjunction with a detector array is also known in the art, as is the use of a mechanical spectral filter selector. For reasons related at least to increased cost, complexity and weight, these conventional approaches to multi-spectral imaging are disadvantageous for many applications.
It was thus realized that the detection of IR radiation in two or more spectral bands with a single integrated detector device was a very desirable alternative to the conventional approaches. Representative examples of such detectors can be found in the following commonly assigned U.S. patents.
In U.S. Pat. No.: 5,113,076 by Eric Schulte, “Two terminal multi-band infrared radiation detector”, there is described a radiation detector that includes a first heterojunction and a second heterojunction that are electrically coupled together in series between a first electrical contact and a second electrical contact. The detector contains at least a three regions or layers, including a first layer having a first type of electrical conductivity, a second layer having a second type of electrical conductivity and a third layer having the first type of electrical conductivity. The first and second heterojunctions are coupled in series and function electrically as two back-to-back diodes. During use the detector is coupled to a switchable bias source that includes a source of positive bias (+Vb) and a source of negative bias (−Vb). With +Vb applied across the detector the first heterojunction is in far forward bias and functions as a low resistance conductor, thereby contributing no significant amount of photocurrent to the circuit. The second heterojunction is in a reverse bias condition and modulates the circuit current in proportion to the photon flux of an associated spectral region or color. Conversely, with −Vb applied across the detector the second heterojunction is in forward bias and contributes no significant photocurrent to the circuit while the first heterojunction is reversed biased and produces a current modulation proportional to the incident flux, where the flux is associated with a different spectral region.
In U.S. Pat. No.: 5,731,621 to Kenneth Kosai, “Three band and four band multispectral structures having two simultaneous signal outputs”, there is described a solid state array that has a plurality of radiation detector unit cells, wherein each unit cell includes a bias-selectable two color photodetector in combination with either a second bias-selectable two color detector or a single photodetector. Each unit cell is thus capable of simultaneously outputting charge carriers resulting from the absorption of electromagnetic radiation within two spectral bands that are selected from one of four spectral bands or three spectral bands.
In U.S. Pat. No.: 5,751,005 by Richard Wyles et al., “Low-crosstalk column differencing circuit architecture for integrated two-color focal plane arrays”, there is described an integrated two-color staring focal plane array having rows and columns of photodetector unit cells, each of which is capable of simultaneously integrating photocurrents resulting from the detection of two spectral bands. A readout circuit performs a subtraction function, and includes a differential charge-sensing amplifier in a one-per-column arrangement. The amplifier works in cooperation with circuitry located in each unit cell. The subtraction function is employed to create a separate Band1 signal from a Band2 and (Band1+Band2) signals generated by each simultaneous two-color detector. The circuit offers low spectral crosstalk between the two spectral bands.
Also by example, in U.S. Pat. No.: 5,959,339 by Chapman et al., “Simultaneous two-wavelength p-n-p-n infrared detector” there is disclosed an array that contains a plurality of radiation detectors. Each radiation detector includes a first photoresponsive diode (D1) having an anode and a cathode that is coupled to an anode of a second photoresponsive diode (D2). The first photoresponsive diode responds to electromagnetic radiation within a first band of wavelengths and the second photoresponsive diode responds to electromagnetic radiation within a second band of wavelengths. Each radiation detector further includes a first electrical contact that is conductively coupled to the anode of the first photoresponsive diode; a second electrical contact that is conductively coupled to the cathode of the first photoresponsive diode and to the anode of the second photoresponsive diode; and a third electrical contact that is conductively coupled to a cathode of each second photoresponsive diode of the array. The electrical contacts are coupled during operation to respective bias potentials. The first electrical contact conducts a first electrical current induced by electromagnetic radiation within the first predetermined band of wavelengths, and the second electrical contact conducts a second electrical current induced by electromagnetic radiation within the second predetermined band of wavelengths, less an electrical current induced by electromagnetic radiation within the first predetermined band of wavelengths.
The disclosures of these various commonly assigned U.S. Patents are incorporated by reference herein in so far as there is no conflict with the teachings of this invention.
Also of interest to the teachings of this invention is a p-i-i-n (p-type, intrinsic, intrinsic, n-type) detector that is described by Brüggermann et al., “The operational principle of a new amorphous silicon based p-i-i-n color detector”, J. Appl. Phys. 81(11), 1 Jun. 1997, 7666–7672. The device is constructed using two large band gap front layers of doped and intrinsic hydrogenated amorphous silicon carbide (a-SiC:H), followed by an intrinsic and a doped a-Si:H layer. These authors report that by band gap engineering an experimental red response is maximized at a large reverse bias voltage, whereas the green response has its maximum at low reverse bias voltage. The potential profile of the p-i-i-n structure is said to be of crucial importance to the color detection mechanism. At larger wavelengths the large potential drop across the two highly defective front layers assists recombination in the back part of the device, which leads to the drop in the red response at low reverse voltage. For the voltage-dependent shift in spectral sensitivity it is said to be important that photogenerated carriers, under green bias illumination, are lost by recombination in the front part of the device.
Also of interest is an n-i-p-i-i-n detector of a type described by H. Stiebig et al., “Transient Behavior of Optimized nipiin Three-Color Detectors”, IEEE Transactions on Electron Devices, Vol. 45, No. 7, Jul. 1998, 1438–1444. These authors report the detection of the fundamental components of visible light (blue, green, red) with a multi-spectral two-terminal photodiode that is based on amorphous silicon. The preferential carrier collection region of the two-terminal device shifts upon a change of the applied bias voltage, which leads to a color sensitivity. Structures with controlled bandgap and mobility-lifetime product exhibit a dynamic behavior above 96 dB. Three linearly independent spectral response curves can be extracted to generate a RGB (red-green-blue)-signal. Bias voltage switching experiments under different monochromatic illumination conditions were carried out to investigate the time-dependent behavior.